1. Field of the Invention
The present invention relates to a new process for the preparation of pimelic esters, by a ring-opening reaction of a salt formed from a 2-oxocyclohexanecarboxylic ester and a strong base.
2. Discussion of the Background
Pimelic esters, as defined in the present application, have the following formula: ##STR2## wherein R.sup.1 and R.sup.2 may be identical or different, and are linear or branched saturated alkyl substituents having from 1 to 10 carbons or araliphatic radicals having 7 to 10 carbon atoms; R.sup.3 is H or a hydrocarbon radical of one to six carbon atoms, and n is an integer of from 0 to 3. Pimelic esters are important intermediates in the preparation of pharmaceuticals, crop protection agents, perfumes or cosmetic active substances, specialty polymers and lubricants.
Pimelic acid has been prepared by a number of different synthetic routes. It is known that pimelic acid can be prepared by partial hydrogenation of salicylic acid--to give tetrahydrosalicylic acid--with subsequent cleavage by sodium hydroxide or potassium hydroxide under drastic conditions; cf. Ullmann, Volume 5, p. 824 (19.54) and Volume 10, p. 140 (1980) and Org. Synthesis Coll. II, pp. 531-38 (1943). Pimelic acid can also be obtained by alkali treatment of cyclohex-4-enecarboxylic acid, which itself can be obtained from butadiene and acrylic acid by a Dieis-Alder reaction [J, Amer,. Chem. Soc. 74, p. 532 (1952)].
Pimelic acid can also be prepared by reaction of alkali metal or alkali metal hydroxides with tetrahydrobenzaldehyde in accordance with German Patent 7331 or with tetrahydrofurylpropanol in accordance with German Patent 2939.
According to the process of EP-A 0 905 651, the synthesis can also be carried out by reduction of 3-oxopimelic ester.
Other synthesis routes start from suberone, which is converted to pimelic acid by oxidative cleavage; cf. U.S. Pat. No. 286,516 and DE-A 20 43 012. The same result is obtained by hydrolysing cyclohexanone-2-carboxamide; cf. DD Patent 112 987.
Pimelic acid is also the result of the oxidation of cycloalkenes with ozone in accordance with DE-A 40 00 163, the hydroformylation of 1,5-pentadiene followed by oxidation in accordance with DE-A 15 18 216, or the carbonylation of .omega.-caprolactone in accordance with U.S. Pat. No. 4,888,443.
However, for many applications it is the esters of pimelic acid which are desired, which have to be prepared from pimelic acid, after the syntheses described above, by esterification according to known methods. These syntheses, which often involve two or more stages, and the subsequent esterification to obtain pimelic esters are highly complex. In addition, none of the known processes provide satisfactory yields and purity combined with ease of manufacture.